The invention relates to a digital data tape reading device. Data is stored by magnetic, optical or magnetic-optical techniques on disks. For large amounts of data, tape is a more convenient storage medium physically. Only magnetic techniques are used currently for tape storage, although other techniques can be envisaged.
In order to read or write data the storage medium (disk or tape) must be moved relatively to a read/write head arrangement. Repositioning with respect to the head arrangement is frequently necessary and in the case of tape devices, repositioning means stopping and restarting the tape. Repositioning causes wear of the tape drive mechanism and should be kept to a minimum. Repositioning is necessitated by differences in the data transfer rate of equipment to or from which the tape data is being supplied. Thus, a computer coupled to a tape storage device may have an inherent data transfer rate which is permanently less than the native transfer rate of data to or from the tape, or the computer data transfer rate may fluctuate because of other demands made on the computer processor.
Temporary fluctuations in relative data transfer rates may be handled by an electronic buffer between the computer and the tape device, the buffer being capable of accepting and feeding out data at differential rates. Permanent differences between the computer data transfer rate and the native tape data transfer rate will cause the buffer to empty or fill, depending on the direction of data flow. Then repositioning would normally become necessary. In order to avoid this when writing to tape it has been proposed to allow the tape to continue to run and to write to the tape, while awaiting the arrival of more data, "amble" tracks which include no data. The "amble" tracks are ignored when the tape is read.
The present invention, on the other hand, addresses the problem of reading data from a tape into equipment having an inherent data transfer rate which is lower than the native data transfer rate of the tape.